Preclinical evidence indicates that dopamine (DA) plays a role in penile erection in mammals. Sexual stimulation can be initiated by sensory (erotic) information reaching the cerebral cortex in mammals. The cerebral cortex has extensive neuronal connections with limbic structures like the amygdala, as well as midbrain structures like the periaqueductal gray (PAG) and the hypothalamus. Two important nuclei in the hypothalamus are the medial preoptic area (MPOA) and the paraventricular nucleus (PVN). The MPOA and PVN nuclei play a critical role in sexual behavior as bilateral lesions of these areas completely eliminate male sexual behavior. The incerto-hypothalamic dopaminergic pathway that innervates the PVN and the MPOA nuclei has been associated with the pro-erectile effect of DA agents. Systemic administration of DA receptor agonists like apomorphine ((6aR) 5,6,6a,7-tetrahydro-6-methyl-4H-dibenzo[de,g]quinoline-10,11-diol), quinpirole and (-) 3-(3-hydroxyphenyl)-N-propylpiperidine (3-PPP) facilitate penile erection in rats, an effect blocked by haloperidol, a central DA antagonist. As the erectogenic effect can not be blocked by domperidone, a peripheral DA antagonist, it is believed that the pro-erectile effect of DA agonists is centrally mediated.
Clinical data also indicates that DA systems in the CNS play a role on the regulation of male sexual behavior as indicated by the sexual stimulatory effect of L-dopa in Parkinson's patients and by the pro-erectile effect of apomorphine in humans.
DA receptors belong to a superfamily of protein receptors that signal across the cell membrane by coupling to intracellular GTP-binding proteins. Several G proteins have been identified (including Gs, Gq and Gi) that lead to specific intracellular events.
There are five known DA receptors which are classified into two groups, D1-like and D2-like. The D1-like receptors include D1 and D5. The D2-like receptors include D2, D3 and D4. The D1-like family receptor subtypes are Gi-coupled and can activate adenylate cyclase. The D2-like family receptor subtypes are Gi-coupled and they increase intracellular calcium level and inhibit adenylate cyclase.
The D1-like family members are Gs-coupled receptors that can activate adenylate cyclase. The D1 receptor is the most abundant and widespread DA receptor in the CNS both by mRNA expression and by immunohistochemical studies. It is found in the striatum, nucleus accumbens and olfactory tubercle as well as the limbic system, hypothalamus and thalamus. The D1 receptor expression has been reported in the heart and kidney, and despite that the function of these peripheral D1 receptors remains to be clarified, its role on the control of hemodynamic variables has been confirmed. The D5 receptor, while having a higher affinity for DA than the D1 receptor, is sparsely distributed in the CNS with no evidence of expression outside the CNS.
The D2-like family members are Gi coupled receptors that inhibit adenylate cyclase and increase intracellular calcium levels. The D2 receptor is the most abundant of the D2-like receptors and is located in brain areas such as the striatum and substantia nigra, and in peripheral areas such as the heart, pituitary gland and kidney. The D3 receptor is found abundantly in the islands of Callej a with distinct cluster populations in the ventral striatum/nucleus accumbens regions, olfactory tubercle, dendate gyrus and striatal cortex.
Expression of the D4 receptor has been documented by in situ RNA hybridization and immunohistochemical studies. Recently, studies revealed that D4 expression is highest in the entorhinal cortex, lateral septal nucleus, hippocampus and the medial preoptic area of the hypothalamus. Localization of D4 is distinct from the distribution of D2 in the brain, as D2 receptors are most abundant in striatal areas. The expression of D4 receptors in the MPOA of the hypothalamus is of importance to the facilitation of penile erection in view of the role of the hypothalamus as an area of integration between the cortex and the spinal pathways. The participation of D4 receptors in other CNS regions, thalamic, subthalamic and spinal can not be excluded.
The present invention identifies a therapeutic use for the compounds of formula (I) in the treatment of sexual dysfunction in mammals. More specifically, these compounds are useful in the treatment of sexual dysfunction including, but not limited to, male erectile dysfunction (MED).